Conventional gas turbine engines often include film cooling and emissions control bypass air. As higher gas turbine engine operating efficiencies are achieved, operating temperatures of the combustion gas approach and may even exceed an acceptable operating temperature for a substrate that forms the structures. In such cases film cooling of surfaces of the structure adjacent the combustion gas (“hot surface”) may be implemented. Often a plurality of holes through the structure permit a portion of the compressed air from a plenum surrounding the combustors to bypass the combustor and flow directly to an interior region of the structure. Once in the interior region all of the compressed air flows unite to form a film between the combustion gas and the hot surface that protects the hot surface from the combustion gas.
Combustion with low NOx and CO emissions levels requires a combustion zone characterized by a uniform flame at a certain temperature. Emissions control bypass air provides a means for optimizing the flame. A relatively hot flame or hotter regions within the flame may produce NOX gas, and a relatively cool flame or relatively cooler regions within the flame may produce CO gas. The flame characteristics may be tuned by adjusting the fuel/air ratio, and this may be controlled by controlling the amount of air that reaches the combustor. Redirecting some of the compressed air from the plenum directly into the structure will adjust the fuel/air ratio because this redirected air simply does not reach the combustor, and therefore is not counted in the fuel/air ratio. For example, when operating at base load, only a small percentage of the plenum air may be redirected from the combustor to ensure there is sufficient air reaching the combustor. Redirecting too much air would decrease the amount of air flowing to the combustor, which would in turn yield a fuel rich (relatively hot) combustion flame that may produce excess NOx emissions. When operating at part load there may be an abundance of air through the combustor, which would yield a fuel lean mixture and therefore a relatively cool flame and associated excess CO emissions. A greater percentage of the plenum air may therefore be redirected when operating at part load to reduce the excess air at the combustor and therefore reduces CO emissions. An example of such a system is disclosed in U.S. Pat. No. 6,237,323 to Ojiro et al.
Conventional gas turbines produce combustion gas traveling at about mach 0.2 to 0.3 within the structure. As a result of the relatively fast moving combustion gas, relatively slow compressed air in the plenum exhibits a higher static pressure than does the fast moving combustion gas within the structure. This pressure difference often drives compressed air from the plenum and through the film cooling apertures. Emerging technology for can annular gas turbine engines include structures that direct combustion gas from combustion to a first row of turbine blades without a need for a first row of vanes to properly orient and accelerate the combustion gas. Structures include the combustor cans themselves together with an assembly that directs combustion gas from the combustor to the first row of turbine blades along a straight flow path at a proper speed and orientation without a first row of vanes. The assembly includes a plurality of flow directing structures, one for each combustor. One such assembly is disclosed in U.S. Pat. No. 7,721,547 to Bancalari et al. issued May 25, 2010, incorporated in its entirety herein by reference.
In both conventional combustors and emerging technology combustors the static pressure exhibited by compressed air in the plenum is approximately the same, and is greater than a static pressure exhibited by the combustion gas 18. Further, for any given set of operating parameters, the pressure difference is constant. Within prior art transition ducts combustion gas typically does not exceed approximately mach 0.2 or 0.3, and therefore exhibits a lower static pressure than the compressed air in the plenum. This pressure difference is sufficient to drive the film cooling circuit. However, unlike prior art transition ducts, in the emerging combustor technology the acceleration geometry 20 accelerates the combustion gas to, for example, mach 0.8. This substantial increase in speed within the flow directing structure 12 yields an associated substantial decrease in static pressure within the combustion gas 18. This in turn provides a much greater pressure difference between the compressed air in the plenum and the combustion gas 18 than in prior art combustion systems. This greater pressure difference is capable of providing much more air to the film circuit than the film cooling circuit needs. Under certain conditions the pressure difference may be so great that a momentum of the flow of cooling air through the film cooling holes is enough to permit the flow to separate from the hot surface. Separating from the hot surface interferes with the formation of the film, and therefore the effectiveness of the film cooling. Efficient cooling schemes are still being developed in conjunction with the emergence of the technology.